A "sidekick" is the hero's partner and helper who works inside and outside of the shadows to support the hero.

Bright Wing, a female hero, and her sidekick Katsuo Hoshino (known as “Toy Bird”) work inside the Hero System—an organization for cracking down on crime perpetrated by people who misuse their special abilities. Katsuo is serving as a sidekick and training to become a hero, but he doesn’t have the special abilities that are an absolute requirement for heroes. Introducing a powerful duo made up of a good-for-nothing woman around age 30 and a busybody man!

(Source: Heros Web)